fix: error in local file path detection

Author: Pfeil

src/main/java/edu/kit/datamanager/ro_crate/util/FileSystemUtil.java

@@ -21,10 +21,10 @@ private FileSystemUtil() {
      * @return true if it looks like a file path, false otherwise
      */
     public static boolean isFilePath(String id) {
-        return !(
-                id.startsWith("doi:") &&
-                        id.startsWith("http") &&
-                        id.startsWith("https")
+        return id != null && !(
+                id.startsWith("doi:") ||
+                id.startsWith("http://") ||
+                id.startsWith("https://")
         );
     }